A multicast communications method is disclosed in published Japanese Patent Application laid-open no. 9252/1988, According to the disclosed method, packet switching is used in a wide-area network (WAN) to interconnect a plurality of widely distributed terminal equipment and communications networks on a customer's premises. By this method, when a packet is transmitted from terminal equipment with a header indicating that the packet is to be transmitted to a plurality of other terminal equipment (multiple terminal equipment), a packet exchange that receives the transmitted packet sends it out to the multiple terminal equipment that are indicated by the transmitted packet. This sets up a point-to-multipoint logical link between the terminal equipment at the transmitting side and the multiple terminal equipment at the receiving side so that the same information is transmitted from one terminal and received by a plurality of other terminals.
In local area networks (LANs) a variety of control information is exchanged between terminals connected on the LAN or between routers used to interconnect LANs on a customer's premises. Usually, this type of control information is exchanged using a multicast communication methods. For example, routers send address information of the terminal equipment to be connected to a LAN and address information for routing to other networks to adjacent routers.
In Japanese Patent Publication laid-open no. 287228/1988, a multicast communications method is disclosed that includes providing destination information in the transmitted data identifying the terminal equipment at the receiving side, providing the node equipment between the sending terminal and the receiving terminal with a table of destination information that indicates the relationship between each piece of terminal equipment and the node equipment or terminal equipment to which the data is transferred next. According to this method, the destination information is compared with the table of destination information, and, if necessary, one or a limited small number of copies of the transmitted data is sent to the particular node equipment or terminal equipment at the receiving side.
When using a packet exchange for performing multicast communications between terminal equipment distributed in a wide area through a WAN, it is required that an additional function of multicast communications be added to the packet exchange. Further, communication by conventional packet exchange has a low through put, so such communication cannot be used to interconnect LANs, which are used on a customer's premises.
In order to provide connection between LANs, a network using an asynchronous transfer mode (ATM) can be used. The ATM network performs switching on the basis of a virtual channel identifier (VCI) that is added to a cell having a fixed length of information, and that is transferred in place of a conventional packet. In an ATM network, it is possible to create a plurality of virtual circuits in a single physical circuit. In this way, multiple LANs can be connected by providing different virtual channel identifiers for each LAN destination. An ATM network, as shown in FIG. 2, has terminal adapters 21A, 21B, 21C, 21D and 21E that connect LANs 12A, 12B, 12C, 12D and 12E to the network. In order to perform multicast communications of a cell to a predetermined number of terminal adapters, copies of the cells must be made and sent out, one for each of the specified number of terminal adapters. Thus, the terminal equipment which can be at the receiving side is limited in number by the multicast capability of the terminal adapters or other node equipment, such as ATM exchanges or routers. Conventional adapters or ATM exchanges perform switching and sending of these cells by hardware. This is necessitated by the higher transmission rate and shorter processing time that is required when performing multicast communications of cells since the cells are shorter in length than the conventional packets. In such high speed processing, the time taken to copy the cells is significant, and therefore only a small number of cells can be copied.
Usually, it is desired to transmit information to only one specific terminal or router, but often the information is multicast or broadcast to all of the terminal adapters because the destination address of the specific terminal or router is not known. Sending information to all of the terminal equipment in the wide area network undesirably increases the traffic in the network. Such an increase in traffic may cause blockage in the network or require that the network be increased in size at an additional cost.